The present invention, in some embodiments thereof, relates to optics and, more particularly, but not exclusively, to an optical element having two phase masks, and to imaging by means of the optical element.
Use of phase masks to extend the Depth of Field (DOF) is known. Several methods for extending the DOF of an imaging system have been published.
Dowski et al., 1995, “Extended depth of field through wave-front coding,” Applied Optics 34, 1859, discloses an optical-digital system which is a standard incoherent optical system modified by a phase mask with digital processing of the resulting intermediate image. The phase mask alters or codes the received incoherent wave front in such a way that the point-spread function and the optical transfer function do not change appreciably as a function of mis-focus.
Wang and Gan, 2001, “High focal depth with a pure phase apodizer,” Applied Optics 40, 5658, disclose use of a pure-phase super resolution apodizers to optimize the axial intensity distribution and extend the DOF of an optical pickup.
Ben-Eliezer et al., 2008, “An optimal binary amplitude-phase mask for hybrid imaging systems that exhibit high resolution and extended depth of field,” OPTICS EXPRESS, 20540, discloses a circularly symmetric hybrid imaging system. A pupil mask provides the largest spatial frequency band that assures a certain desired contrast value for a certain desired depth of field.
Also known is the use of combination of two diffractive optical elements. For example, S. M. Ebstein, 1996, “Nearly index-matched optics for aspherical, diffractive, and achromatic-phase diffractive elements,” Optics Letters, Vol. 21, No. 18, 1454, discloses nearly index-matched optical elements that have application to aspherical, diffractive, and hybrid refractive-diffractive elements; and Arieli et al., 1998, “Design of a diffractive optical element for multiple wavelengths,” Applied Optics, 37, No. 26, 6174, discloses a method for producing diffractive optical elements from two different optical materials, taking advantage of their different refractive indices and dispersions.